This invention relates to a master cylinder having a primary piston fixed to an output push rod retained in a movable wall of a brake booster and moved to a position of rest in a bore of the master cylinder by a return spring which moves the movable wall of the brake booster to a position of rest position in the brake booster in an absence of an operator input being applied to a control valve to effect a brake application. With the primary piston in the position of rest, a supply of fluid retained in a reservoir is made available to compensate for any fluid loss in a brake system.
In tandem master cylinders, a single bore in a housing usually retains a primary piston and a secondary piston. A first spring arrangement separates the primary piston from the secondary piston to define a first chamber within the bore while a second spring arrangement separates the secondary piston from a end wall of the bore to define a second chamber. An input force applied to the primary piston after overcoming the secondary spring arrangement moves the primary piston, first spring arrangement and secondary piston to close communication between compensation passages and the bore and thereafter pressurize fluid in the first and second chambers. The pressurized fluid is supplied to the wheels brakes to effect a brake application. On termination of the input force on the primary piston, the first and second spring arrangements move the primary and secondary pistons in a position of rest. In this position of rest, communication is established between a fluid supply retained in a reservoir and the first and second chambers to compensate for any losses of fluid that may occur in a brake system. The following U.S. Pat. Nos. 3,149,468, 5,161,375, 5,279,125, and 5,943,863 are illustrative examples of the above described master cylinder.
The master cylinders disclosed in the above identified patents function in an adequate manner to provide pressurized fluid to a brake system for effecting a brake application. However, recent efforts have been made to reduce the cost of a master cylinder through the use of light weight metal materials for the master cylinder housing and plastic material for the primary and secondary pistons. Unfortunately such changes have not reduced the number of components needed to manufacture a master cylinder and as a result the time of manufacture a master cylinder did not change.
A primary object of the present invention is to eliminate a caged spring assembly which separates a primary piston from a secondary piston in a master cylinder by fixing the primary piston of the master cylinder to an output push rod of a brake booster and utilize a return spring which moves a movable wall of the brake booster in a positions of rest to locate the primary piston in the master cylinder and define a primary pressure chamber within the bore while opening communication between the bore and primary pressure chamber in a position of rest.
In more particular detail, the brake system for a vehicle includes the brake booster which provides the primary piston in the master cylinder with an input force to pressurize fluid in a first or primary pressure chamber. This pressurized fluid is supplied to wheel brakes of a vehicle to effect a brake application in response to an operator input force. The master cylinder has a first housing with a bore therein having corresponding first and second compensation ports connected to a fluid supply in a reservoir and first and second outlet ports connected to the wheel brakes. The primary piston is located in the bore and with a secondary piston defines the first pressure chamber there between while a spring urges the secondary piston into engagement with a stop to define a second pressure chamber therein. With the primary and secondary pistons in a position of rest, the first and second chambers are connected through compensation ports to the fluid supply available in the reservoir to compensate the brake system with any fluid loss that may occur in the brake system. The brake booster has a second housing with an interior separated by a movable wall into a front chamber and a rear chamber. The front chamber is connected to a first fluid which has a first pressure (vacuum) while the rear chamber is selectively connected to the first fluid and to a second fluid at a second pressure (atmospheric pressure) as a function of an input force applied a control valve. A return spring located in the second housing urges the movable wall toward a position of rest whereby the first fluid pressure (vacuum) is communicated to the front and rear chambers through the control valve. The control valve is responsive to an operator input for terminating communication of the first fluid pressure to the rear chamber and initiating communication of the second fluid to the rear chamber to create an pressure differential across the movable wall. The pressure differential acts on the movable wall to develop an operational force, which after overcoming the return spring, moves the movable wall from the position of rest toward the master cylinder. The operational force is communicated from the movable wall through a reaction member and into the output push rod to provide the primary piston with an operational input. The primary piston moves from its position of rest past to interrupt communication between the bore and compensation port and thereafter pressurize fluid in the first pressure chamber. The pressurized fluid from the first pressure chamber is supplied from the first pressure chamber to wheel brakes through a first outlet port to initiate a brake application. The pressurized fluid present in the first pressure chamber also acts on the secondary piston and after overcoming its return spring pressurizes fluid in the second pressure chamber. The pressurized fluid from the second pressure chamber is communicated through a second outlet port to wheel brakes to assist in effecting the brake application. The brake system is characterized a connecting means which joins the primary piston of the master cylinder with the output push rod of the brake booster such that the primary piston is moved toward its position of rest by the return spring returning the movable wall of the brake booster to its position of rest in an absence of an operator input being applied to the control valve.
An advantage of the brake system resides in the master cylinder and brake booster combination wherein a return spring for the brake booster moves both a primary piston in the master cylinder and movable wall of the booster to a position of rest.
A further advantage of the brake system resides in a connection whereby a primary piston in a master cylinder is fixed to an output rod of a movable wall in a brake booster and movement of the wall in respond to a pressure differential across the movable wall simultaneously moves the primary piston to pressurize fluid in a master cylinder to effect a brake application.
A still further advantage of the brake system resides in fixing a primary piston in a master cylinder to an output push rod of a brake booster to tie movement of the primary piston with the movement of a movable wall in the brake booster.